The Socialist Unity Party of Germany (German: Sozialistische Einheitspartei Deutschlands, shortened: SED) was the governing party of East Germany from 1949, when East Germany was created, until the elections of 1990.

A famous SED poster read: To learn from Stalin means to learn how to win. Later, the word Stalin was replaced with the USSR. When Gorbachev was president of the USSR the famous slogan was abandoned, as the SED did not believe there was anything to learn from his ideas about perestroika.

At first the SED had a branch in West Berlin, but in 1962 the West Berlin branch was separated from the SED proper and became a "separate" party called the Socialist Unity Party of West Berlin (Sozialistische Einheitspartei Westberlins – SEW) but with the same ideas as the parent party.

Between the fall of the Berlin Wall in November 1989 and the election in March 1990, the old Social Democratic Party was re-established as a separate party, while the rest of the SED lost a lot of members and renamed itself the Party of Democratic Socialism (PDS) at a special party meeting in December 1989. By becoming the PDS the party survived the reuniting of East Germany and West Germany, and eventually started growing again, managing to get representatives elected to the Bundestag.

On December 1, 1989 the SED was forced to give up its 40 year monopoly of power when the State Council (Volksammer) revoked the clause in the constitution of the GDR that guaranteed the SED to be the sole legal party in East Germany. Its leader and all of the politburo resigned three days later and the party was finally dissolved on December 16, 1989.

These three party leaders were also the equivalent of the Prime Minister or President in other countries; they really led the government as well as the main, legally powerful party, which was not often the case in other Communist countries.